target/riscv: Add missing include guard in pmu.h

Add missing include guard in pmu.h to avoid the problem of double
inclusion.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240220110907.10479-1-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/pmu.h

@@ -16,6 +16,9 @@
  * this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#ifndef RISCV_PMU_H
+#define RISCV_PMU_H
+
 #include "cpu.h"
 #include "qapi/error.h"
 
@@ -31,3 +34,5 @@ int riscv_pmu_incr_ctr(RISCVCPU *cpu, enum riscv_pmu_event_idx event_idx);
 void riscv_pmu_generate_fdt_node(void *fdt, uint32_t cmask, char *pmu_name);
 int riscv_pmu_setup_timer(CPURISCVState *env, uint64_t value,
                           uint32_t ctr_idx);
+
+#endif /* RISCV_PMU_H */